Fiber optics is increasingly becoming the transport medium of choice for high-speed data communications. The VCSEL with its relatively low cost and high intensity output has made fiber optics readily available for use in local area networking applications. VCSELs however are typically multi-mode devices and are typically used with low cost multi-mode fibers, for example, plastic optic fibers.
The light output from a VCSEL must be efficiently coupled into an optic fiber. For this to occur, the beam divergence must be not greater than the acceptance angle of the fiber. A typical VCSEL has a farfield twin-peak output characteristic as shown in FIG. 1. In FIG. 1, the y-axis represents the power output and the x-axis represents the viewing angle. The different curves represent different output powers. It will be observed from FIG. 1 that at the highest output power, the VCSEL produces maximum output at angles of about 7.5° and 10° off the axis. There is a substantial dip in the power output along the axis. The total divergence of the output beam is about 26° for the highest output power, which is greater than the acceptance angle of the optic fiber. This makes coupling of the light output into an optic fiber inefficient because clearly if the optic fiber is aligned with the optical axis it will not receive the maximum output power, and if the alignment is optimized for one lobe it will not receive much power from the other. The off-axis alignment of the output power also encourages less efficient multi-mode propagation in the optic fiber.
A further problem arises from the fact that the light output distribution changes with changing output powers of the VCSEL. For example, it will be observed that as the output power is reduced, the left peak shifts to the right. In FIG. 1, the second highest output power has a left peak at about 5°. This further complicates the design of the coupling into the optic fiber.
The changing spatial distribution of output power also has another undesirable effect. Most VCSELs are provided with a power monitor in the form of a PIN diode that receives light reflected off the output lens of the device. If the angular spatial intensity distribution changes with output power, the power monitoring becomes non-linear because the intensity seen by the PIN diode is affected not only by the reduction in output power, but also by the fact that the intensity in the angular direction that is reflected back to the power monitor changes with drive current.